The fluidized bed boilers related to the present invention and, more accurately, the components thereof, are equipped with so-called water/steam tube walls, which literally consist of parallel water/steam tubes and plate-like fins between the tubes. The purpose of the water or steam circulating in the water/steam tubes is, depending on the application, mainly to recover heat generated in combustion, or to maintain the wall temperature within reasonable values. As for their strength, such water/steam tube walls are, in view of their size, relatively weak, so they cannot be used to bear directly the stresses caused by the weight of the boiler or of the components thereof, the pressure of the combustion space or thermal expansion, perhaps notwithstanding the direction of the tubes, but, for example, different beam structures are used to support the boiler and different structural parts thereof.
Mainly due to thermal expansion, large fluidized bed boilers, with all of their structural elements, are suspended to hang from the supporting structures of a building housing the boiler. In other words, both the furnace and the separators, removing solid material from the flue gases and attached thereto, have conventionally been mounted by means of hanger rods or wires to the supporting structures of the boiler building. More accurately, in constructions in accordance with the prior art, the furnace has been suspended, in the manner described above, directly to the supporting structures, but mainly, two methods have been used in supporting the separators, depending on how the separator and the flue gas channel passing the flue gases forward therein are arranged relative to each other. An arrangement used more often is to provide each separator with a flue gas channel of its own leading, when viewed from the direction of the furnace, away from the separator. In this case, the separator can be suspended to hang directly from its walls to primary steel structures above the separator. This has been generally realized by arranging hanger rods to the circumference of the separator at intervals of 400 mm, which results in using fifteen to twenty-five hanger rods, naturally depending on the size of the separator. Another method used, especially in large fluidized bed boilers to arrange a flue gas channel in communication with a separator, is, first of all, to arrange separators on opposite sides of the furnace and to arrange a common flue gas channel for the separators to both sides of the furnace. The flue gas channel is naturally situated above the separators. The separators of this arrangement are suspended to the supporting structures by means of the flue gas channel located above them. In principle, the structure is quite applicable. However, with the increase in the size of the power boilers and, at the same time, of the separators, and with the change in their shape, a number of problems have arisen.
First of all, the separators typically used with the fluidized bed boilers are vortex separators in their operational principle, and, traditionally, they have been circular in their cross section. During the last decade, the shape of these separators started to change to quadrate, whereby their mounting to the flue gas channel became easier. There, it was possible to hang the separators either (1) from all sides to the supporting structure above, in other words, to a so-called primary steel structure by means of hanger rods, or, (2) if the separator was first assembled to the flue gas channel, it was possible to use the whole wall of the flue gas channel as a supporting line. Recently, the cross-sectional shape of the separators has become, to a certain extent, rounder, and octagonal separators have become really popular in the market. With this shape, especially with large separators, it is no longer possible to support the separator to the primary steel structure, nor to the bottom of the flue gas channel. When supporting the separator directly to the primary steel structure, the problem will be that there are a number of walls that are in angular position relative to the primary steel structure, lacking a beam above in the primary steel structure, and thus, a place where to hang the separator.
A fact in practice in joining the separators to the flue gas channel, which, in fact, has already been referred to above, is that it is not possible to arrange the bottom of the flue gas channel to support heavy separators, but the only possible supporting areas in the flue gas channel are the vertical sidewalls thereof. It has been noted, however, that large, not right-angled separators do not have enough load carrying areas (lines) between the circumference of the separator and the wall line of the flue gas channel. In practice, this means that, for example, one third or one fourth of the circumferential length of the separator (depending on whether the separator is hexagonal or octagonal) supports the weight of the whole separator and the sand and ash therein, which, in turn, leads to too heavy of local stresses. Exactly in the same way, when the separators change to non-rectangular, only part of the sidewalls of the flue gas channel directly bears the weight of the separators. Theoretically, with hexagonal separators, about 50% of the length of the sidewall of the flue gas channel in maximum, and with octagonal separators, only about 40% of the length of the sidewall of the flu gas channel, bears the load directly.
Secondly, since large boilers, practically speaking, always have separators on both sides of the furnace, the flue gas cleaned by the separators should be taken from the separators via as short a route as possible to the heat recovery section of the fluidized bed boiler, and to minimize the construction costs and the pressure losses. On the one hand, this means that the heat recovery section must be arranged as an extension of the furnace on one of its two sides left free and, on the other hand, that the flue gases from both sides of the furnace must be brought to the heat recovery section via as short a route as possible. The result of this is that the flue gas channels must be brought above the separators to the end of the separator bank, and drawn further to the heat recovery section.
It has been noted that one of the problems with this arrangement is that the maximum width of the flue gas channel with the present separator supporting methods may be equal to the diameter of the separator. In some arrangements, even a smaller width of the flue gas channel has been used, but it has proved especially problematic, because a fact is that the cross section of the flue gas channel has to be large enough, so that the flue gases can flow without any substantial pressure loss in the heat recovery section. In other words, if the width of the flue gas channel is limited, the required cross section must be gained by increasing the height of the channel. This, however, instantly increases the total height of the building. The same problem is to be seen in cases where the width of the flue gas channel is limited to the diameter of the separator, especially, when talking about large boilers, whereby there are a lot of separators on both sides of the furnace. This naturally leads to a need for a large cross-sectional flow area.
Thirdly, it has been impossible to define the weight of the separator, and also the weight of sand and ash therein, when using the constructions in accordance with the prior art, by measuring, because there have always been a large number of hanger rods. They have sometimes been positioned between the flue gas channel and the supporting structures, too, whereby the flue gas channel has, in a way, tied all of the separators to one package.
An additional problem related to a large number of hanger rods is that it has been, in practice, very difficult, if not impossible, to arrange the supporting of the separator in such a way that all hanger rods are subject to the same load. Rather, in conventional suspension, some of the rods carry very little weight, whereas others already yield under the weight.
Thus, it is an object of the invention to arrange the supporting of the separator in such a way that the harmful effects of the above-mentioned problems can be diminished.